1. Field of the Invention
This invention relates to a plasma torch comprising an electrode secured to a liquid-cooled electrode holder and formed with a flow passage communicating with a central outlet for delivering an ionizable gas and a nozzle body which surrounds said electrode and serves to conduct gas along the outside surface of the electrode.
2. Description of the Prior Art
Compared to a plasma torch having a solid electrode, a plasma burner having an electrode which is formed with a centrally disposed flow passage for supplying a part of the ionizable plasma flame affords the advantage that the centrally supplied plasma gas effects an additional cooling. It is known from DE-A-31 41 476 that long, stable electric arcs can be produced by such plasma torches if an annular nozzle orifice is provided between the frustoconical electrode and a coaxial nozzle body, which surrounds the electrode, and said orifice directs the plasma gas into the electric arc at an acute angle thereto. That orifice is so shaped that the gas flows out in such a direction that the stability of the arc is substantially improved. On the other hand, a disadvantage resides in that the special shape of the nozzle orifice causes the electrode to be axially set back relative to the nozzle body so that the nozzle body is subjected to a high thermal load, which causes the nozzle body to be rapidly consumed. As a result, the geometry of the nozzle orifice will be altered and the flow at the desired angle may not be maintained for a prolonged time. Besides, the electrodes have only a restricted current-carrying capacity.
From DE-B-1 954 851 it is known that the electric power and the efficiency of a plasma beam generator can be increased in that the velocity of the plasma jet leaving the nozzle is increased. For this purpose the outlet nozzle of the arc discharge chamber of the plasma jet generator consists of a double nozzle and the inner outlet orifice and the annular outer outlet orifice constitute respective Laval nozzle passages. A disadvantage of that known plasma jet generator resides in that the design of the outlet orifice precludes an increase of the torch power because the plasma jet which has been formed in the arc discharge chamber is present adjacent to the outlet nozzle.